The present invention relates to electronic flash units, and more particularly, to electronic flash units of the dynamic flat flash type in which a flash discharge tube intermittently emits pulse flashes of an extremely short time duration so as to make them substantially equivalent to a continuous flash of a constant intensity.
As is well known, an intensity of flashlight of a flash discharge tube in an electronic flash unit generally has a peaked configuration which rapidly rises upon its initiation and falls in an extremely short time (see characteristic curve S.sub.0 in FIG. 1).
With a camera having a focal plane shutter, it is impossible for an electronic flash unit to emit flashlight at a higher shutter speed than a flash synchronized shutter speed, resulting in a failure of a normal synchro-flash photography. Specifically, when a higher shutter speed than the flash synchronized shutter speed is used a focal plane shutter does not fully open and while a slit formed by first and second shutter blinds is running in front of a film surface only part of the film surface is exposed to flashlight so that a uniform exposure over the entire film surface can not be expected.
To overcome this disadvantage, there has already been provided an electronic flash unit which emits a continuous flash of constant intensity (a characteristic curve S.sub.1 in FIG. 1) while the slit is running in front of the film surface (hereinafter referred to as a static flat flash type electronic flash unit).
The static flat flash type electronic flash unit, which, for example, is disclosed in Japanese Laid-Open Patent Application No. 129327/1980, has a basic circuitry in which a serially connected circuit of a flash discharge tube, an inductor and a switching element is connected across a main capacitor in which flash emission energy is stored and a diode is connected in parallel with a series circuit formed by the discharge tube and the inductor. When a flashlight intensity of the discharge tube decreases below a predetermined level by monitoring the flashlight intensity the switching element turns on and when the flashlight intensity exceeds the predetermined level the switching element turns off so that the flash emission can be continued at a substantially constant light intensity. However, the conventional static flat flash type electronic flash unit should be so controlled that flashlight emitted from the discharge tube is maintained at an intermediate intensity between a preset upper limit of flashlight intensity and a lower limit thereof extremely close to the upper limit. Accordingly, an extremely precise comparator is required and the necessary circuitry is complicated. Moreover, there is a possibility of malfunction.
In addition, the on-off control of the switching element is performed, with an impedance element such as a resistor connected in series with the discharge tube, by detecting a discharge current of the discharge tube which also flows through the impedance element and controlling a flashlight intensity based on a current value is thus detected. Accordingly, loss in the discharge current is caused by the impedance element, as well as an accompanying loss in a quantity of the emitted flashlight. This results in that a change in the flashlight intensity does not accord with that in the discharge current so that it is impossible to effect a precise flash control.
In addition, when the flashlight intensity is controlled based on a voltage value at a terminal of the discharge tube which is detected directly, the detected terminal voltage fluctuates as the switching element turns on and off, so that it is difficult to effect a precise flash control. When a coil for preventing an overcurrent is added in series with the discharge tube, an increased oscillating voltage is produced by turning on and off the switching element.
In addition, the conventional static flat flash type electronic flash unit is for an exclusive use in the flat flashlight emission and is not used for a normal electronic flash, especially an automatic flash control type electronic flash unit.
The conventional static flat flash type electronic flash unit has an opening on part of a reflecting shade for illuminating a predetermined portion of an object to be photographed by flashlight, on which opening a photoelectric transducer element such as a photodiode and a phototransistor is arranged so that a flashlight intensity of the discharge tube can be detected based on an output signal of the transducer element. However, since the output signal of the transducer element is of an extremely low level, an electric circuit for dealing with the output signal is susceptible to extraneous noises, resulting in malfunctions by a trigger signal of a high voltage which triggers the discharge tube. The high voltage trigger signal is transmitted through a stray capacity and the like to the transducer element and hence there is a possibility of causing deterioration of the transducer element and malfunctions.
With the conventional static flat flash type electronic flash unit, a quantity of emitted flashlight is made variable, as disclosed in Japanese Patent Publication No. 40421/1973, by changing a voltage across terminals of the discharge tube. However, since variation in flashlight intensity with respect to the voltage across the discharge tube terminals is nonlinear, a complicated adjustment circuit is required.
On the other hand, as for a light quantity of the artificial illumination for photography, a guide number (GN) with a film speed value 100 in ISO is used as the common numerical value. Although there are various theories on a coefficient correction between a guide number and a flashlight quantity L (cd.s), the following relation holds in principle; ##EQU1## where K represents a correction coefficient. However, when a slit exposure is effected with a camera employing a focal plane shutter, only part of a flashlight quantity contributes to an actual exposure of a film surface so that a GN value is not properly indicated as the total flashlight quantity.
Accordingly, it is necessary to deal with only part of the flashlight quantity which contributes to actual exposure of the film surface through a slit as an effective guide number GNe with a film speed ISO 100. However, since the effective guide number GNe varies with a slit exposure time or a slit width as a parameter, a photographer should bear this in mind while taking a picture. In addition, when an aperture-priority automatic exposure camera is used, an exposure time automatically varies in accordance with brightness of an object to be photographed and an effective guide number GNe varies accordingly.